1. Field of the Invention
The present invention relates to geophysical exploration, and more particularly to seismic surveying and processing of seismic data and surface-consistent refraction analysis for deriving near surface corrections of the seismic data.
2. Description of the Related Art
The seismic industry has been trending towards dramatic increases in the number of the seismic acquisition channels. The increased availability of data which are acquired in seismic surveys with increased acquisition channels has made available the recording of full azimuth data at an unprecedented spatial resolution.
Conventional seismic data processing and analysis methods that were developed some several years ago are being found unsuitable to handle the large amount of data provided by modern seismic acquisition systems. One area of seismic processing chain that is showing difficulties to cope with the increased size of the seismic datasets is near surface analysis. For example, analysis of the near surface in search of subtle anomalies has been a problem. These subtle anomalies can have significant impact in the imaging of low relief structures and of stratigraphic traps.
So far as is known, pressing methodology for the purpose of analysis of the near surface is still based on interactive procedures where the input of the analyst is required. An example of processing requiring input by an analyst before analysis of the near surface is the estimation of refraction arrivals through semi-automated first break (FB) picking which requires time-consuming human intervention to quality control the data.
High resolution velocity analysis for the near surface could be provided by full-waveform inversion (FWI) methods. However, full wave inversion of the increased size seismic datasets requires large amounts of computational power, in fact often unmanageable amounts when considering the case of three dimensional (3D) elastic wave analysis. Moreover, full wave inversion processing of the increased size seismic datasets has exhibited sensitivity to noise and free surface effects such as surface waves. Full wave inversion processing of this type also requires seismic waves at low frequencies to avoid the effects of what are known as cycle skips. In addition, full wave inversion processing has required good velocity models as a starting point in order to limit the non-linearity of the problem.
Seismic tomography methods, commonly used in seismic data processing, whether for near surface or other purposes, also show difficulties in adapting to the volumes of data and need to compromise between the density of information and a viable parameterization of the models. Non-linear inversion methods are computationally demanding in that a single computer processing run of the massive amounts of data requires several inner iterations, and also several computer processing runs are required for exploring the parameter space and the initial model space.
There have been increasing numbers of seismic data channels becoming available from sampling with large redundancy every few meters of the subsurface, resulting in several billions of traces. However, seismic processing methods currently in use to analyze the near surface are not able to take full advantage of the redundancy of the information provided. So far as is known, effective processing of the amount of information, to provide high resolution velocity models for sharp and accurate near surface corrections, has not been available.
One alternative approach of near surface correction, used with the conditions of dense subsurface sampling and large redundancy, is based on adoption of broad-sense statistical methods. Under certain acceptable simplifying assumptions based on the physics of the problem, these have offered robust, quick and effective solutions to seismic imaging. One example of this sort is the well-known collection of surface-consistent algorithms for processing seismic reflection data. In this context, the term surface-consistent are defined to mean that a given location on the earth surface is associated with a fixed time delay without regard to the total wave path. This type of processing has been effectively used in everyday processing work. Calculation of surface-consistent reflection residual statics is a fundamental step in land seismic data processing. Surface-consistent residual statics techniques of this type solve the short wavelength/high frequency component of near surface correction to improve the coherency of reflected events, and enhancing the imaging of the subsurface geology. Reflection statics are based on cross-correlation of high frequency seismic signals. For this reason, preventive application of long wavelength refraction statics is required align the traces and avoid cycle skipping. In a conventional seismic processing workflow the long wavelength component of the static solution is obtained through the laborious analysis and inversion of refracted data.
The adoption of surface-consistent approaches for the analysis of first break of first break data has been proposed as a methodology for taking advantage of the redundancy provided by high-channel count seismic acquisition systems. Statistical analysis in a surface-consistent fashion of large volumes of data was envisioned as a potential solution to robust quality control of first break travel times and for reduction of overall turnaround time for near surface analysis.
It has also been proposed to incorporate first break data in an existing surface-consistent deconvolution procedure. The surface-consistent deconvolution processing could permit derivation of attribute maps, such as scaled velocities, under certain conditions. The conditions were that these attribute maps had to correlate well with the expected geology, and exhibit sealed travel times which matched the general behavior of the input data.